Back-reflection protection and monitoring in fiber and fiber-delivered lasers

ABSTRACT

A system includes an optical fiber situated to propagate a laser beam received from a laser source to an output of the optical fiber, a first cladding light stripper optically coupled to the optical fiber and situated to extract at least a portion of forward-propagating cladding light in the optical fiber, and a second cladding light stripper optically coupled to the optical fiber between the first cladding light stripper and the optical fiber output and situated to extract at least a portion of backward-propagating cladding light in the optical fiber.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 14/816,211, filed Aug. 3, 2015, which claims the benefit of U.S. Provisional Application No. 62/032,043, filed on Aug. 1, 2014, both of which are hereby incorporated by reference in their entirety.

FIELD

The disclosure pertains to back-reflection protection in fiber and fiber-delivered laser systems.

BACKGROUND

High-power industrial laser systems generally produce beams having output powers in the range of several hundreds of Watts to several kW. It is often desirable to deliver the laser power to a processing head or work piece via an optical fiber. Laser systems that can be coupled into an optical fiber for delivery include fiber lasers, disk lasers, and diode- or lamp-pumped solid-state lasers (e.g., Nd:YAG). In these systems, the desired optical power is guided in the fiber core, but some power may also be present in the fiber cladding; this cladding light is undesirable because it can cause excessive heating of or damage to downstream components or optics, or it may otherwise interfere with work piece processing.

In typical fiber laser systems a signal beam is created in an active fiber that includes a rare-earth doped optical fiber core by delivering a pump beam to a cladding of the active fiber at a pump wavelength that is shorter than a signal beam wavelength. At the output of the fiber laser, cladding light may consist of unabsorbed pump light and signal light that has escaped from the core. For both fiber lasers and other lasers, cladding light may be introduced into a beam delivery fiber if launching of the laser beam couples some of the light into the cladding rather than the core. A processing beam directed to a target or work piece can experience reflection or scattering and can become back-coupled into the beam delivery fiber. This back-coupled light can be coupled into both the active or beam delivery fiber core and cladding, and can destabilize, damage, or otherwise interfere with the laser system.

Accordingly, systems that can remove and monitor both forward-propagating and backward-propagating cladding light and backward-propagating core light are needed, particularly to protect against high-power back-reflections.

SUMMARY

According to one aspect a system includes an optical fiber situated to propagate a laser beam received from a laser source to an output of the optical fiber, a first cladding light stripper optically coupled to the optical fiber and situated to extract at least a portion of forward-propagating cladding light in the optical fiber, and a second cladding light stripper optically coupled to the optical fiber between the first cladding light stripper and the optical fiber output and situated to extract at least a portion of backward-propagating cladding light in the optical fiber.

According to another aspect, a method includes directing a material processing laser beam from a laser source from an output end of a beam delivery fiber to a target, receiving a returned portion of the material processing laser beam returned from the target, coupling the returned portion into the beam delivery fiber, extracting cladding light associated with the material processing laser beam in a first cladding light stripper optically coupled to the beam delivery fiber, and extracting cladding light associated with the returned portion of the material processing laser beam in a second cladding light stripper optically situated between the first cladding light stripper and the output end of the beam delivery fiber.

According to another aspect, a method includes detecting a characteristic associated with backward-propagating cladding light extracted with a cladding light stripper from a beam delivery fiber delivering a laser beam generated with a laser source, detecting a characteristic associated with backward-propagating core light in the beam delivery fiber, and adjusting one or more laser system characteristics in response to one or more of the detected backward-propagating cladding light and backward-propagating core light characteristics.

According to another aspect, a system includes a first cladding light stripper disposed between a laser source and a beam delivery fiber output situated in relation to a target, the first cladding light stripper situated to receive a material processing beam and to extract forward-propagating cladding light associated with the material processing beam from the beam delivery fiber, and a second cladding light stripper disposed between the first cladding light stripper and the beam delivery fiber output and situated to receive the material processing beam from the first cladding light stripper and to receive backward-propagating cladding light associated with the target, the second cladding light stripper situated to extract backward-propagating cladding light from the beam delivery fiber.

According to another aspect, a laser feedback monitoring system in a high-power laser system is provided, the high-power laser system configured to produce a laser beam from a laser source and to emit the laser beam from a delivery fiber output and to direct the emitted beam to a target, with the laser feedback monitoring system including a first cladding light stripper situated between the laser source and the delivery fiber output to receive the laser beam and to remove cladding light associated with the beam, a second cladding light stripper situated between the first cladding light stripper and the delivery fiber output to receive the beam from the first cladding light stripper and to remove residual cladding light associated with the beam and to receive light back-reflected from the target and coupled back into the delivery fiber output and to remove back-reflected cladding light, and at least one back-reflected cladding light detector coupled to the second cladding light stripper for detecting characteristics of the back-reflected cladding light.

The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an embodiment of a laser system with back-reflection protection.

FIG. 2 is a schematic of another embodiment of laser system with back-reflection protection.

FIG. 3 is a schematic of another embodiment of a laser system with back-reflection protection.

FIG. 4 is a schematic of another embodiment of a laser system with back-reflection protection.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” does not exclude the presence of intermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation.

Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.

In some examples, values, procedures, or apparatus' are referred to as “lowest”, “best”, “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections. Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” and the like. These terms are used for convenient description, but do not imply any particular spatial orientation.

As used herein, optical radiation refers to electromagnetic radiation at wavelengths of between about 100 nm and 10 μm, and typically between about 500 nm and 2 μm. Examples based on available laser diode sources and optical fibers generally are associated with wavelengths of between about 800 nm and 1700 nm. In some examples, propagating optical radiation is referred to as one or more beams having diameters, asymmetric fast and slow axes, beam cross-sectional areas, and beam divergences that can depend on beam wavelength and the optical systems used for beam shaping. For convenience, optical radiation is referred to as light or beams in some examples, and need not be at visible wavelengths. Forward-propagating light or optical beams or beam portions refer to light, beams, or beam portions that propagate in a common direction with a processing beam that is directed to a target. Backward-propagating light or optical beams or beam portions refer to light, beams, or beam portions that propagate in a common and opposite direction of a processing beam that is directed to a target.

Representative embodiments are described with reference to optical fibers, but other types of optical waveguides can be used having square, rectangular, polygonal, oval, elliptical or other cross-sections. Optical fibers are typically formed of silica (glass) that is doped (or undoped) so as to provide predetermined refractive indices or refractive index differences. In some, examples, fibers or other waveguides are made of other materials such as fluorozirconates, fluoroaluminates, fluoride or phosphate glasses, chalcogenide glasses, or crystalline materials such as sapphire, depending on wavelengths of interest. Refractive indices of silica and fluoride glasses are typically about 1.5, but refractive indices of other materials such as chalcogenides can be 3 or more. In still other examples, optical fibers can be formed in part of plastics. In typical examples, a doped waveguide core such as a fiber core provides optical gain in response to pumping, and core and claddings are approximately concentric. In other examples, one or more of the core and claddings are decentered, and in some examples, core and cladding orientation and/or displacement vary along a waveguide length.

As used herein, numerical aperture (NA) refers to a largest angle of incidence with respect to a propagation axis defined by an optical waveguide for which propagating optical radiation is substantially confined. In optical fibers, fiber cores and fiber claddings can have associated NAs, typically defined by refractive index differences between a core and cladding layer, or adjacent cladding layers, respectively. While optical radiation propagating at such NAs is generally well confined, associated electromagnetic fields such as evanescent fields typically extend into an adjacent cladding layer. In some examples, a core NA is associated with a core/inner cladding refractive index, and a cladding NA is associated with an inner cladding/outer cladding refractive index difference. For an optical fiber having a core refractive index n_(core) and a cladding index n_(clad), a fiber core NA is NA=√{square root over (n² _(core)−n² _(clad))}. For an optical fiber with an inner core and an outer core adjacent the inner core, a cladding NA is NA=√{square root over (n² _(inner)−n² _(outer))}, wherein n_(inner) and n_(outer) are refractive indices of the inner cladding and the outer cladding, respectively. Optical beams as discussed above can also be referred to as having a beam NA which is associated with a beam angular radius. While multi-core step index fibers are described below, gradient index designs can also be used.

In the examples disclosed herein, a waveguide core such as an optical fiber core is doped with a rare earth element such as Nd, Yb, Ho, Er, or other active dopants or combinations thereof. Such actively doped cores can provide optical gain in response to optical or other pumping. As disclosed below, waveguides having such active dopants can be used to form optical amplifiers, or, if provided with suitable optical feedback such as reflective layers, mirrors, Bragg gratings, or other feedback mechanisms, such waveguides can generate laser emissions. Optical pump radiation can be arranged to co-propagate and/or counter-propagate in the waveguide with respect to a propagation direction of an emitted laser beam or an amplified beam.

The term brightness is used herein to refer to optical beam power per unit area per solid angle. In some examples, optical beam power is provided with one or more laser diodes that produce beams whose solid angles are proportional to beam wavelength and beam area. Selection of beam area and beam solid angle can produce pump beams that couple selected pump beam powers into one or more core or cladding layers of double, triple, or other multi-clad optical fibers.

Referring now to FIG. 1, a high-power laser system with feedback protection 10 includes a laser source 12 situated to couple an optical beam into a delivery fiber 16 as a guided optical beam that exits the deliver fiber 16 as an output beam 14. The output beam 14 is typically directed to a target 18 such as a metal, glass, semiconductor or other material substrate. A first cladding light stripper 20 is optically coupled to the delivery fiber 16 so as to remove, or strip, primarily forward-propagating cladding light (i.e., light propagating towards the target 18) from the guided beam. A second cladding light stripper 22 is optically coupled between the first cladding light stripper 20 and the output of the delivery fiber 16 so as to remove, or strip, primarily backward-propagating cladding light (i.e., light propagating form the output of the delivery fiber 16 toward the laser source 12). The forward-propagating cladding light is generally associated with unabsorbed or unconverted pump light and portions of the output beam that has leaked out of a core of the delivery fiber 16. The backward-propagating cladding light is generally associated with portions of the output beam 14 that are back-coupled into the delivery fiber 16 due to scattering, reflection, or other effects.

A returned beam portion 24 that is coupled into the output of the delivery fiber 16 is generally undesirable and can destabilize the laser system 10 or cause laser system degradation and failure. By arranging a pair of cladding light strippers, such as the first and second cladding light strippers 20, 22, laser systems such as the laser system 12 can be provided with robust protection from back-reflection. In particular, the second cladding light stripper 22, which is more proximal to the output of the delivery fiber 16 as shown in FIG. 1, substantially removes portions of the output beam 14 that are coupled into the delivery fiber 16 as cladding light, while forward-propagating cladding light associated with the output beam 14 and pump beams is substantially attenuated by the first cladding light stripper 20, situated more distal from the output of the delivery fiber 16 than the second cladding light stripper 22. This removal of cladding light allows a portion of the delivery fiber 16 between the first and second cladding light strippers 20, 22 to be substantially without cladding light. Thus, the backward-propagating cladding light can be isolated or mostly isolated, and therefore distinguishable, from the forward-propagating cladding light.

The cladding light strippers 20, 22 can also be conveniently arranged in close proximity to each other and still retain optical or thermal isolation. For example, in some embodiments, the cladding light strippers 20, 22 can be separated and separately or jointly cooled or otherwise thermally managed. In other embodiments, the cladding light strippers 20, 22 are disposed on a common heat sink block 26 which is actively cooled with one or more cooling devices coupled to the heat sink block 26. Depending on the design of the cladding light strippers 20, 22, forward-propagating cladding light may also be partially removed by the second cladding light stripper 22, and backward-propagating cladding light may also be partially removed by the first cladding light stripper 20. Forward-propagating cladding light is generally undesirable because it can cause heating or damage to components downstream from the fiber laser, e.g., to the delivery cable or to optical systems employed to deliver the laser beam to the work piece.

Depending on the output power of the output beam 14 (which can be as little as 0.5 kW or less to as much as 10 kW or more) and other attributes of the high-power laser system 10, various forward-propagating cladding light and back-coupled light power contents can be expected during operation. For example, the laser system 10 configured to generate the output beam 14 with an output power of around 3 kW can include 300 W or more of forward-propagating cladding light that should be coupled out of the delivered beam 14 and dissipated to prevent system destabilization. Theoretically, up to the entirety of the output beam 14 could be back-reflected and coupled back into the output of the delivery fiber 16, though 100% back-coupling may be unlikely. With the 3 kW output beam 14, in typical examples approximately 300 W can be expected to become back-coupled, however the portion of the output beam 14 that is back-reflected, the variability of the portion, and the time dependence of the portion can be dependent on the application in which the high-power laser system output beam 14 is being used. Accordingly, back-coupled light and backward-propagating cladding light can range from being continuous to highly transient. In some system examples, 100 W or more of continuous forward-propagating cladding light is removed and 100 W or more of transient backward-propagating cladding light is removed. The cladding light strippers 20, 22 are thus operable to remove the cladding light and the removed light can be dumped in some manner, such as with a conductive sink like conductive block 26.

Referring now to FIG. 2, a high power laser system with feedback protection 40 includes a laser source 42 that generates a fiber-coupled laser beam 44 which is typically a high-power beam for material processing applications, and a delivery fiber 46 that directs the beam 44 to a target 48. A first cladding light stripper 50 is situated to remove a portion (e.g., 80%, 90%, 95%, 99%) or all forward-propagating cladding light from the beam 44 as the forward-propagating cladding light propagates towards the output of the delivery fiber 46. A second cladding light stripper 52 is located in the beam path between the first cladding light stripper 50 and the output of the delivery fiber 46 so as to remove a portion (e.g., 80%, 90%, 95%, 99%) or all backward-propagating cladding light that is typically associated with returned beam portions 54 from the target 48. If less than the entire amount of forward-propagating cladding light is removed by the first cladding light stripper 50, then the second cladding light stripper 52 can remove all or most of the remaining portion. Similarly, if less than the entire amount of backward-propagating cladding light is removed by the second cladding light stripper 52, the first cladding light stripper 50 can remove all or most of the remaining portion. The cladding light strippers 50, 52 are conveniently coupled to a conductive block 56 which can be configured for active cooling.

The laser system 40 also includes a stripped forward-propagating cladding light detector 58 coupled to the first cladding light stripper 50. The detector 58 is situated to detect one or more characteristics associated with the forward-propagating cladding light, such as power, wavelength, temporal characteristics (e.g., power variation, pulse duration, pulse repetition rate), temperature in proximity to the stripper 50, etc., and provides an associated detection signal to a system controller 60. The detector 58 can include one sensor for detecting a particular characteristic, one sensor for detecting a plurality of characteristics, or a plurality of sensors for detecting the same or different characteristics. In some examples, the first cladding light stripper 50 includes a volume formed in the conductive block 56, such as an integrating volume. The forward-propagating cladding light propagates along a fiber situated in the volume, such as a portion of the beam delivery fiber 46 or another fiber or fiber portion coupled to the beam delivery fiber 46. One or more sensors associated with the detector 58 can be situated in or near the volume so as to generate detection signals to detect forward-propagating cladding light characteristics. To moderate detected characteristics of cladding light, one or more optical filters may be used. Detected characteristics can also include specular or diffuse reflections of cladding light extracted from the optical fiber in which the forward-propagating cladding light is propagating.

The laser system 40 further includes a stripped backward-propagating cladding light detector 62 coupled to the second cladding light stripper 52 which detects one or more characteristics associated with the backward-propagating cladding light, such as power, wavelength, temporal characteristics, temperature, etc. In some examples, the detector 62 (and/or detector 58) can include one or more of a thermistor, thermal switch, and photodiode. The output of the detector 62 is also coupled to the controller 60. Because most or all of the forward-propagating cladding light associated with the beam 44 is removed by the forward-propagating cladding light stripper 50, the detector 62 is situated to detect back-reflected characteristics without the characteristics becoming tainted, skewed, or muddled by the forward-propagating cladding light energy. Similarly, since most or all of the backward-propagating cladding light energy is removed by the backward-propagating cladding light stripper 52, the process of detecting forward-propagating cladding light characteristics experiences improved accuracy in the presence of back-reflected light.

The system controller 60 is situated to receive one or more such detection signals that can be associated with corresponding beam characteristics and can initiate different system actions in response. For example, controller 60 is shown to be coupled with control output 64 to laser source 42. Based on the detected characteristics from detectors 58, 62, a laser interlock procedure can be executed that triggers a power disconnect or an adjustment of power level or some other characteristic associated with the laser system 40, including the laser source 42 and the output beam 44. For example, if the backward-propagating cladding light power or an associated temperature is excessive, the laser source 42 can be turned off, or a warning can be issued to the user. Abnormal increases or decreases of the forward-propagating cladding power may indicate a malfunction of the laser system, which can result in turning off the laser source 42 or issuing a warning to a user. In other examples, laser source 42 output power can be varied to compensate for a detected variation in forward-propagating cladding energy. Other laser system characteristics can be adjusted as well, including cooling system characteristics (e.g., fluid flow-rates, power level, etc.).

The cladding light strippers 50, 52 typically couple only cladding light, or poorly coupled cladding light, out of the optical delivery fiber 46. Both the forward- and backward-propagating core light associated with beam propagation in the delivery fiber 46 is generally unaffected. Backward-propagating core light can also lead to system degradation and failure, and the system controller 60 can be coupled to a backward-propagating core light detector 66, that is coupled to the laser source 42 and detects one or more characteristics associated with backward-propagating core light. The interlock control 64 of the controller 66 can also be configured to disconnect or adjust power or another laser system characteristic associated with the laser source 42. In some embodiments, detected cladding light and core light characteristics can include spectral characteristics, allowing deeper insights into system operation and particular application effects, and enabling a spectral-based interlock for the system 40 or spectral-based adjustment of laser system characteristics, including output beam power, pump source power, and temporal features.

In some examples, a detector 68 is coupled to the optical fiber 46 propagating the output beam 44 between the first and second cladding light strippers 50, 52. Because most or all of the forward and backward-propagating cladding light is removed by the strippers 50, 52, the length of optical fiber 46 extending between the strippers 50, 52 provides a location suitable to detect characteristics of core light associated with the propagating output beam 44 such that detection experiences less interference associated with forward or backward cladding light energy. In one example, a core light power characteristic is detected with detector 68 coupled to a core light detection chamber 70 without interfering with propagating beam 44 by receiving light scattered out of the core. The light detection chamber 70 can serve as an optical integrating volume such that detector 68 receives a multiply reflected, integrated characteristic of the scattered core light. In another example, the detector 68 is coupled so as to directly receive the scattered core light or one or more mirrors or lenses can be used to concentrate and direct the scattered core light to the detector 68. In a further example, an optical splice is disposed on the optical fiber 46 in the detection chamber 70. The detector 68 can then detect optical loss associated with the propagation of light through the optical splice, and such loss can vary in relation to the optical power and wavelength of the beam 44 propagating in the core of the fiber. It will be appreciated that various features of detectors 58, 62, 68 as well as the detected characteristics can be used together.

In various examples, values of particular detected optical characteristics can provide the actual or close to the actual characteristic sought, or the values can be empirically or observationally matched to different expected or actual characteristics. For example, the laser system can be operated at selected power levels and an output beam can be measured both in the presence and absence of cladding light strippers or other system components to determine the amount of cladding light that is removed and corresponding detected values. A table can be generated, and a function fitted to the table values, associating the different detected characteristics in relation to the actual characteristics.

Cladding light strippers can be made in a variety of ways. Some examples of cladding light strippers utilize an epoxy that surrounds an exposed cladding surface of an optical fiber in which an output beam and cladding light are propagating. Because the fiber cladding is coupled to an epoxy with selected refractive index and other characteristics, cladding light is successively stripped along the length of the epoxy-cladding interface. In other examples of cladding light strippers, the optical fiber includes one or more notches or other patterns (“microstructures”) penetrating the circumference of the exposed cladding. The notches are operable to disrupt the propagating cladding light by directing the cladding light away from and out of the optical fiber without substantially affecting the light propagating in the fiber core. The out-coupled cladding light then impacts an adjacent surface of a conductive block and is eventually converted to heat, which dissipates through the conductive block. In other examples of cladding light strippers, silica-based crystals are formed on the cladding surface to scatter the cladding light out of the fiber without substantially affecting the light propagating in the fiber core.

Referring to FIG. 3 there is shown another example of a high power laser system 100 with back-reflection protection. System 100 includes laser source 102 capable of generating a high-power fiber-coupled laser beam 104, which is emitted from an output of a delivery fiber 106 towards a target 108. System 100 further includes first and second cladding light strippers 110, 112 disposed on a cooling block 114 and in relation to the output of the delivery fiber 106. The first cladding light stripper 110 removes all or most of forward-propagating cladding light typically associated with residual pump light and signal light lost from the core of the laser source 102, while second cladding light stripper 112 removes all or most of backward-propagating cladding light typically associated with light 116 reflected at the target and back-coupled into the output of the delivery fiber 106.

Laser source 102 is expanded to show an example of a laser source herein suitable for generating the high power fiber-coupled output beam 104. It will be appreciated that a variety of types and variations of laser sources are possible that can be configured to produce a fiber-delivered high power output beam, including laser sources configured to detect different operating system characteristics and to receive different control commands. Other examples of suitable laser sources include fiber lasers, fiber amplifiers, disk lasers, diode-pumped solid-state lasers, lamp-pumped lasers, and direct-diode lasers, each of which can be configured to operate at a wavelength that can be transmitted to the processing head or work piece using an optical fiber.

As shown, the exemplary laser source 102 includes a plurality of pump modules 118 that produce pump beams at a pump wavelength and couple the pump beams into a pump delivery fiber 120. The plurality of pump delivery fibers 120 is coupled to a pump or pump-signal combiner 122 which combines received pump light into a combined pump output 124. The output 124 is coupled to a fiber laser oscillator 126 that generates the output beam 104 in an active core of the fiber oscillator using the coupled pump light. The pump delivery fibers 120 are arranged in various configurations about an input of the combiner 122. An additional fiber 128 is coupled to a center position of the input of the combiner 122 and can be coupled to one or more components, including an aiming laser 130, a detector 132, or a beam dump 134. The aiming laser 130 emits an aiming beam which can assist with the use of the system 100 by showing the expected location for the beam 104 at the target 108. The center position occupied with the additional fiber 128 is also convenient for detection and removal of core light that is not contained by the oscillator 126. In particular, back-reflected light 116 that is coupled into the core of the delivery fiber 106 may not be removed by the cladding light strippers 110, 112 and can propagate through to the laser source 102.

The core detector 132 is thus situated to detect characteristics associated with core light propagating past the combiner 122 and to provide the detected characteristics to a system controller 136. Based in part or in whole on the received characteristics, the controller 136 can adjust other system characteristics, such as the power level supplied to the pump modules 118 using controller output 138, in order to protect or enhance system 100 performance. Moreover, the central position of the fiber 128 of the combiner 122 ensures most or all of the core light received by the combiner 122 from the direction of the oscillator 126 is directed into the fiber 128, which can then be detected with core light detector 132 and removed with beam dump 134. A detector 140 which detects forward-propagating cladding light is coupled to the first cladding light stripper 110 and provides information about forward-propagating cladding light characteristics to the controller 136. A detector 142 which detects backward-propagating cladding light is coupled to the second cladding light stripper 112 and provides information about backward-propagating cladding light characteristics to the controller 136. Independently or in concert, the detectors 132, 140, 142 can detect beam and system characteristics that can be utilized to optimize, protect, or otherwise enhance performance of laser system 100.

In FIG. 4 an exemplary laser system 400 that emits a combined laser beam 402 is shown. The laser system 400 includes laser sources 404 a, 404 b each situated to produce a respective forward-propagating laser beam 406 a, 406 b coupled to a respective receiving fiber 408 a, 408 b. The forward-propagating laser beams 406 are coupled to a beam combiner 410 that combines and couples the forward-propagating laser beams 406 into an output fiber 412 that emits the combined laser beam 402. The combined laser beam 402 is incident on a target 414 and a beam portion 416 associated with the beam 402 is typically reflected by the target 414. Some of the beam portion 416 that becomes reflected can couple into the output fiber 412 and backward-propagate in the receiving fibers 408. Forward propagating cladding light strippers 418 a, 418 b are optically coupled to the respective receiving fiber 408 so as to extract at least a portion of forward-propagating cladding light, such as unused pump light associated with the respective laser sources 404. Backward-propagating cladding light strippers 420 a, 420 b are optically coupled to the respective receiving fiber 408 so as to extract at least a portion of backward-propagating cladding light, such as light that is coupled back into the output fiber 412. The cladding light strippers 418, 420 are coupled to respective conductive blocks 422 a, 422 b so as to dissipate the heat associated with the extracted cladding light. In further examples, the strippers 418, 420 can be coupled to the same cooling block. In additional examples, the forward-propagating cladding light strippers 418 can be situated in a common volume associated with a cooling block and the backward-propagating cladding light strippers 420 can be situated in a common volume associated with a cooling block. In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only representative examples and should not be taken as limiting the scope of the disclosure. Alternatives specifically addressed in these sections are merely exemplary and do not constitute all possible alternatives to the embodiments described herein. For instance, various components of systems described herein may be combined in function and use. We therefore claim all that comes within the scope and spirit of the appended claims. 

I claim:
 1. A system, comprising: an optical fiber situated to propagate a laser beam received from a laser source to an output of the optical fiber; a first cladding light stripper optically coupled to the optical fiber and situated to extract at least a portion of forward-propagating cladding light in the optical fiber; and a second cladding light stripper optically coupled to the optical fiber between the first cladding light stripper and the optical fiber output and situated to extract at least a portion of backward-propagating cladding light in the optical fiber.
 2. The system of claim 1, further comprising: a heat sink situated to receive and absorb the forward-propagating cladding light and the backward-propagating cladding light extracted by the first and second cladding light strippers, respectively.
 3. The system of claim 1, further comprising: a detection chamber optically situated between the first and second cladding light strippers.
 4. The system of claim 1, further comprising: a beam directing optical system situated to receive the laser beam from the output of the optical fiber.
 5. The system of claim 4, wherein the beam directing optical system includes an output beam combiner situated to combine the received laser beam with one or more other laser beams.
 6. The system of claim 1, wherein less than about 10% of the forward-propagating cladding light and less than about 10% of the backward-propagating cladding light propagates in the optical fiber between the first and second cladding light strippers.
 7. A method, comprising: directing a material processing laser beam from a laser source from an output end of a beam delivery fiber to a target; receiving a returned portion of the material processing laser beam returned from the target; coupling the returned portion into the beam delivery fiber; extracting cladding light associated with the material processing laser beam in a first cladding light stripper optically coupled to the beam delivery fiber; and extracting cladding light associated with the returned portion of the material processing laser beam in a second cladding light stripper optically situated between the first cladding light stripper and the output end of the beam delivery fiber.
 8. The method of claim 7, further comprising: detecting a characteristic associated with backward-propagating cladding light extracted with the second cladding light stripper; detecting a characteristic associated with backward-propagating core light propagating in the beam delivery fiber; and adjusting one or more laser system characteristics in response to one or both of the detected backward-propagating cladding light or the backward-propagating core light characteristics.
 9. The method of claim 8, wherein the one or more adjusted laser system characteristics includes a power level of one or more pump beams associated with the laser source.
 10. A system, comprising: a first cladding light stripper disposed between a laser source and a beam delivery fiber output situated in relation to a target, the first cladding light stripper situated to receive a material processing beam and to extract forward-propagating cladding light associated with the material processing beam from the beam delivery fiber; and a second cladding light stripper disposed between the first cladding light stripper and the beam delivery fiber output and situated to receive the material processing beam from the first cladding light stripper and to receive backward-propagating cladding light associated with the target, the second cladding light stripper situated to extract backward-propagating cladding light from the beam delivery fiber.
 11. The system of claim 10, further comprising: a core light detector situated between the first and second cladding light strippers to detect a material processing beam core light power level.
 12. The system of claim 11, wherein the core light detector is situated to receive core light associated with an optical splice that is positioned between the first and second cladding light strippers.
 13. The system of claim 10, further comprising: a detector coupled to the second cladding light stripper and situated to detect one or more characteristics associated with the backward-propagating cladding light.
 14. The system of claim 13, wherein the characteristics associated with the backward-propagating cladding light include one or more of temporal, spectral, thermal, and power characteristics.
 15. The system of claim 13, further comprising: a laser interlock coupled to the detector so as to receive the characteristics associated with the backward-propagating cladding light from the detector and responsive to adjust characteristics of the material processing beam based on the characteristics associated with the backward-propagating cladding light.
 16. The system of claim 13, wherein the detector includes one or more of a thermistor, thermal switch, and photodiode.
 17. The system of claim 10, further comprising: a combiner associated with the laser source and including a central input and one or more other inputs adjacent to the central input; a core light detector optically coupled to the central input of the combiner and situated to detect backward-propagating core light.
 18. The system of claim 17, further comprising a laser system aiming beam coupled to the central input of the combiner.
 19. The system of claim 17, further comprising a beam dump coupled to the central input of the combiner.
 20. The system of claim 17, wherein the combiner is a pump or pump-signal combiner. 